Although photovoltaic systems have become increasingly cost-effective, they have not begun to significantly displace fossil fuels in the United States or elsewhere. Market forces will drive the adoption of PV systems if the installed cost per watt falls below the price of fossil fuels. Approximately half of the installed cost of PV systems comes from the PV modules and the remainder from installation, electronics and land costs. Although less-expensive and less-efficient PV device technologies can reduce the module cost, they often do not significantly alter the “balance-of-systems” cost and can, in fact, increase the land costs. Minimally expensive approaches to high-efficiency PV are therefore of significant value.
Photon upconversion (UC) is a process in which the sequential absorption of two or more lower-energy photons leads to the emission of a single photon with higher energy than any of the absorbed photons. Photon UC can enable low-cost high-efficiency solar energy harvesting and can be useful in both single- and multi-junction photovoltaic devices. Photon upconversion also has applications in military operations, energy efficient lighting, information gathering and security operations.
Photon upconversion has been previously demonstrated, but photon upconversion cannot alter the economics of PV devices and other technologies unless it is highly efficient. There are two important metrics of efficiency: 1) probability of upconversion and 2) emitted photon energy. The probability of upconversion is determined by the average number of low energy photons absorbed for every high energy photon emitted. Upconversion probability efficiency is maximized when all absorbed low energy photons are converted into emitted high energy photons, e.g., no more than two low energy photons are absorbed for every high energy photon emitted. Photon energy efficiency is limited by energy conservation; the emitted photon must have energy less than or equal to the sum of the energy of all incident photons that participate in the upconversion process. Photon energy efficiency is maximized when the emitted photon has energy equal to the sum of the incident photons. Device applications, including PV devices, benefit from maximizing both emitted photon energy and upconversion probability.
Existing approaches to upconversion have relatively low efficiency. Nonlinear upconversion (e.g. frequency doubling) proceeds via virtual intermediate states. Although photon energy conversion efficiency is maximized by frequency doubling, the probability of upconversion is extremely low and is not relevant for solar photon fluxes. Auger upconversion processes are inefficient because they require the simultaneous presence of two electron-hole pairs (excitons), require radiative emission (loss) of one absorbed low-energy photon, and typically upconvert by only a few hundred meV. Thus both the upconversion probability and photon energy efficiency of Auger processes is very low. Approaches based on lanthanide-doped NaYF4 and other similar existing materials upconvert by a process with similarly low photon energy efficiency.
U.S. Pat. No. 8,093,488 discloses a hybrid photovoltaic cell using amorphous silicon germanium absorbers with wide band-gap dopant layers and a photon up-converter. This document discloses use of quantum dots of a single material for upconversion. Efficient photon upconversion has proven difficult with single quantum dot materials because there is no path to achieving high upconversion probability.
Published U.S. Patent Application No. 2010/0288344 discloses methods and apparatus for wavelength conversion in solar cells and solar cell covers by upconverting two low-energy photons into one high-energy photon and by down-converting one high energy photon into two low-energy photons. The document discloses absorber/emission layers doped by rare earth elements or comprised of multi-quantum well structures. The structures disclosed in Application No. 2010/0288344 have a limited maximum efficiency because the energy loss pathways are very challenging to suppress.
Maximizing the probability of upconversion requires minimizing radiative and nonradiative loss pathways for electrons and holes generated by absorbed low-energy photons. Some sacrifice of photon energy is required to minimize these loss pathways and maximize the probability of upconversion. High net upconversion efficiency requires a device that maximizes the probability of upconversion while minimizing the photon energy that must be sacrificed. Thus, there is a need for an approach that maximizes net upconversion efficiency.